Update Moodle app related language strings to reflect the new restrictions and Moodle apps plans

An initial list of language strings that needs to be updated to reflect/indicate admins that they may be affected by site restrictions.

• Remote CSS
• Disabled features, custom menu items, custom lang strings
• $string['requestaccesskey'] = 'Request access key';
• $string['sitemustberegistered'] = 'In order to use the public Airnotifier instance, your site must be registered with Moodle.';
• $string['errorretrievingkey'] = 'An error occurred while retrieving the access key. Your site must be registered to use this service. If your site is already registered, please try updating your registration.';
• $string['keyretrievedsuccessfully'] = 'Key retrieved successfully';
• $string['moodleapp'] = 'Moodle app';
• $string['moodleapp_help'] = '<p>If you have an account on a Moodle site, you can access all your courses on your mobile devices with our free mobile app for Moodle.</p>
• <a href="https://moodle.com/mobile-app">Get the Moodle app</a>';